1. Field of the Invention
The present invention relates to a semiconductor integrated circuit including a functional block such as a RAM (Random Access Memory), a logic section connected to the functional block, and a test circuit for testing them.
2. Description of Related Art
FIG. 21 is a circuit diagram showing a configuration of a conventional semiconductor integrated circuit including a scan test function disclosed in a Relevant Reference 1. As shown in FIG. 21, the semiconductor integrated circuit includes selectors 10, 11 and 12 controlled by a shift mode signal SM; flip-flops (FFs) 30, 31 and 32; selectors 50, 51 and 52 controlled by a test mode signal TEST; logic sections 80 and 81; and a RAM 91.
In FIG. 21, the selectors 10, 11 and 12 and flip-flops 30, 31 and 32 constitute a scan path. The scan path is a memory circuit including parallel paths across the outputs of the logic section 80 and the inputs of the RAM 91, and a serial shift path for serially transmitting data from an SI (scan-in) terminal to an SO (scan-out) terminal.
Next, the operation of the semiconductor integrated circuit as shown in FIG. 21 will be described.
In a normal operation mode, the selectors 10, 11 and 12 are switched to their “0” input terminals by placing a shift mode signal at SM=0, and the selectors 50, 51 and 52 are switched to their “0” input terminals by placing a test mode signal at TEST=0. Thus, the data output from the logic section 80 are selected by the selectors 10, 11 and 12 to be supplied to the input terminals DI0, DI1 and DI2 of the RAM 91 via the flip-flops 30, 31 and 32. Although not shown in this figure, the flip-flops 30, 31 and 32 are supplied with a clock signal. In addition, the data from the output terminals DO0, DO1 and DO2 of the RAM 91 are selected by the selectors 50, 51 and 52 to be delivered to the logic section 81. In this way, in the normal operation mode, the data write and read are carried out under the condition that the RAM 91 is interposed between the logic sections 80 and 81.
In the scan test mode of the logic sections 80 and 81, the selectors 50, 51 and 52 are switched to the “1” input terminals by placing the test mode signal at TEST=1. In this state, the selectors 50, 51 and 52 select and output the data fed to the “1” input terminals. Accordingly, the RAM 91 is bypassed under the condition that the scan path is interposed between the logic section 80 and logic section 81. In this state, the scan test of the logic sections 80 and 81 is carried out with controlling the shift mode signal SM.
In the scan test mode of the logic section 81, the selectors 10, 11 and 12 are switched to the “1” input terminals by placing the shift mode signal at SM=1 so that they select the data fed to the “1” input terminals. Accordingly, when the flip-flops 30, 31 and 32 are supplied with three clock pulses, 3-bit test data fed to the SI terminal are shifted serially and stored in the flip-flops 30, 31 and 32. Since the test mode signal TEST=1 in this case, the 3-bit test data stored in the flip-flops 30, 31 and 32 are supplied to the logic section 81. Thus, the scan test of the logic section 81 is carried out by checking the data the logic section 81 outputs.
In the scan test mode of the logic section 80, the selectors 10, 11 and 12 are switched to the “0” input terminals by placing the shift mode signal at SM=0 so that they select the 3-bit data output from the logic section 80, which has received test data and carried out specified operation. Receiving one clock pulse, the flip-flops 30, 31 and 32 store the 3-bit data fed from the logic section 80. The 1-bit data stored in the flip-flop 32 is output from the SO terminal. Subsequently, the selectors 10, 11 and 12 are switched to the “1” input terminals by placing the shift mode signal at SM=1. Then, supplying the flip-flops 30, 31 and 32 with two clock pulses causes the 1-bit data stored in the flip-flops 30 and 31 to be shifted and output serially from the SO terminal, thereby implementing the scan test of the logic section 80.
The semiconductor integrated circuit as shown in FIG. 21 can set the test data from the SI terminal to the input terminals DI0, DI1 and DI2 of the RAM 91 by the serial shift operation while the shift mode signal SM=1. However, it cannot load the data output from the output terminals DO0, DO1 and DO2 of the RAM 91 onto the flip-flops 30, 31 and 32 to output the data from the SO terminal. Consequently, it cannot carry out the test of the RAM 91 in isolation.
FIG. 22 is a circuit diagram showing a configuration of a conventional semiconductor integrated circuit with the test function of the RAM 91 in isolation, which is disclosed in the Relevant Reference 1. To carry out the test of the RAM 91, it includes, in addition to the semiconductor integrated circuit as shown in FIG. 21, selectors 60, 61 and 62 controlled by an output selecting signal SELDO, and selectors 70, 71 and 72 controlled by a RAM test signal RAMTEST.
The selectors 60, 61 and 62 have their “1” input terminals supplied with the data from the output terminals DO0, DO1 and DO2 of the RAM 91. The selector 60 has its “0” input terminal supplied with the test data from the SI terminal, and selectors 61 and 62 have their “0” input terminals supplied with the data from the flip-flops 30 and 31, respectively. On the other hand, the selectors 70, 71 and 72 have their “0” input terminals supplied with the data from the flip-flops 30, 31 and 32, and have their “1” input terminals with the RAM test data from the SID terminal.
Next, the operation of the semiconductor integrated circuit as shown in FIG. 22 will be described.
In the normal operation mode, the selectors 10, 11 and 12 are switched to their “0” input terminals by placing the shift mode signal at SM=0, the selectors 50, 51 and 52 are switched to their “0” input terminals by placing the test mode signal at TEST=0, and the selectors 70, 71 and 72 are switched to their “0” input terminals by placing the RAM test signal at RAMTEST=0. In this state, the data output from the logic section 80 are supplied to the input terminals DI0, DI1 and DI2 of the RAM 91 via the flip-flops 30, 31 and 32. The flip-flops 30, 31 and 32 are fed with the clock signal. The data from the output terminals DO0, DO1 and DO2 of the RAM 91 are transferred to the logic 81. Thus, in the normal operation mode, the data are written and read in the condition that the RAM 91 is interposed between the logic sections 80 and 81.
In the scan test mode of the logic sections 80 and 81, the selectors 50, 51 and 52 are switched to their “1” input terminals by placing the test mode signal at TEST=1, and the selectors 60, 61 and 62 are switched to their “0” input terminals by placing the output selecting signal at SELDO=0. Thus, the RAM 91 and the scan path are place in the condition that the RAM 91 is by passed, and the scan path is interposed between the logic sections 80 and 81. In this state, the logic sections 80 and 81 are subjected to the scan test by controlling the shift mode signal SM in the same manner as the semiconductor integrated circuit as shown in FIG. 21.
To test the RAM 91, the selectors 70, 71 and 72 are switched to their “1” input terminals by placing the RAM test signal at RAMTEST=1 so that the RAM test data from the SID terminal is supplied to the RAM 91 as the write data. Here, the 1-bit RAM test data is supplied to the RAM 91 in common as the 3-bit write data. In other words, the write data such as “000” or “111” are simultaneously supplied to the RAM 91.
The selectors 60, 61 and 62 controlled by the output selecting signal SELDO are provided for the purpose of loading the test result data from the output terminals DO0-DO2 of the RAM 91 onto the scan path. When the selectors 60, 61 and 62 are switched to their “1” input terminals by placing the output selecting signal at SELDO=1, and the selectors 10, 11 and 12 are switched to their “1” input terminals by placing the shift mode signal at SM=1, a clock pulse applied to the flip-flops 30, 31 and 32 causes them to store the test result data from the output terminals DO0-DO2 of the RAM 91. In this case, the 1-bit data stored in the flip-flop 32 is output from the SO terminal. Subsequently, the selectors 60, 61 and 62 are switched to their “0” input terminals by placing the output selecting signal SELDO=0, and two clock pulses are applied to the flip-flops 30, 31 and 32. Thus, the 1-bit data stored in the flip-flops 30 and 31 are read out of the SO terminal by the serial shift operation. Thus, a test device outside the chip or a self-test circuit inside the chip makes a fault decision.
Relevant Reference 1: U.S. Pat. No. 5,960,008 (particularly, from column 5, line 12 to column 7, line 59).
With the foregoing configuration, the conventional semiconductor integrated circuit as shown in FIG. 21 has a problem of being unable to carry out the test of the functional block such as the RAM 91 in isolation. In addition, the circuit as shown in FIG. 22 has a problem in that the scale of the test circuit of the functional block such as the RAM 91 inevitably increases.